Motor vehicles typically include a bumper and a fascia, i.e. a covering placed over the bumper to improve the vehicle's appearance and/or aerodynamics. Vehicle bumpers are designed to absorb the energy of a low speed collision so as to avoid damage to the rest of the vehicle. Thus, in the event of such a collision, only the bumper and the fascia would need to be replaced, thereby lowering repair costs. Space between the fascia and the bumper is typically filled with foam.
In a low speed collision, for example at 5 miles per hour, a bumper will be expected to absorb a given amount of energy. The amount of energy (e) to be absorbed will be defined by the mass (m) and speed (v) of the vehicle, according to the formula e=(0.5)(m)(v2). Thus, in a test of a given vehicle at a given speed, the energy to be absorbed (in bringing the vehicle to a stop) will be constant. The energy absorbed by a bumper is a function of the “stroke” (s), which is the distance the vehicle continues to move after contacting the test barrier before coming to a stop, and the force (F) applied in stopping the vehicle, according to the formula e=(F)(s). Therefore, a higher force and lower stroke may correspond to the same energy absorption as a lower force and a higher stroke. However, if the force is too great, excess energy will be transferred to the vehicle frame, resulting in damage thereto, and if the stroke is too great, there may be damage to portions of the vehicle other than the bumper. Thus, force and stroke must be balanced, and vehicle manufacturers set standards for both stroke and force for a given vehicle. These requirements are referred to herein as “performance requirements”.
One factor that can influence a bumper's performance is the area moment of inertia (IA) at various points along the bumper beam. The area moment of inertia is a measure of a structural member's stiffness and strength. For a given amount of a particular type of material, the area moment of inertia at a given position is a function of the cross-sectional shape of the structure at that point. For a force applied perpendicularly to a face of a solid rectangular section, IA=(a3×b)/12 where a is the depth of the rectangle in the direction in which the force is applied, and b is the height of the rectangle in a direction perpendicular to the application of the force. For a solid cylinder, IA=(π×r4)/4 where r is the radius, and for a hollow cylinder IA=(π×(a4−b4))/4, where a is the radius of the outer surface and b is the radius of the inner surface. For more complicated shapes, the calculations are more complex. A higher area moment of inertia corresponds to a greater resistance to bending.
In addition to meeting performance requirements, a bumper must also meet “layout requirements”. In other words, a bumper must be able to be securely mounted to the side rails of a vehicle, and the location and configuration of the side rails may vary from vehicle to vehicle. Furthermore, there will be limits imposed on the dimensions of the bumper, as it will be required to fit between the ends of the side rails and behind and under the vehicle fascia. In addition, a vehicle manufacturer may set weight requirements and cost requirements for a bumper.
Traditionally, bumpers have been manufactured by roll forming, i.e. metal forming by using contoured rolls. When the roll forming process is used to form a bumper, sheet metal is bent into the desired shape, and the longitudinal edge is then welded together. Bumpers made by the roll-forming process are formed from a continuous sheet of metal, and will emerge from the manufacturing process as a continuous, usually curved, tube that is then cut into segments of appropriate size. Therefore, when roll forming is used to produce a bumper, it typically results in a constant radius of curvature for the bumper as a whole. In other words, the radius of curvature of a notional centroid path through the tube will be constant, where the notional centroid path is defined as a path upon which any center point of a vertical cross section taken at any position on the bumper element will lie. In addition, roll forming alone generally produces a bumper having a constant cross-section, in the absence of further manufacturing steps. Having such a constant radius of curvature and constant cross section imposes limitations on the design of the bumper. As a result, roll formed bumpers do not always have the design flexibility needed to adjust to the performance requirements and/or layout requirements of modern motor vehicles.
U.S. Pat. No. 6,349,521 to McKeon et al. discloses a hydroformed bumper beam having a non-constant cross-section in that the front and rear walls comprise arcuate center sections having different radii. This type of construction is said to produce a bumper beam having a high energy absorbing but flexible center section.
However, there remains a need for a bumper beam having an impact region shaped to optimize the ability of the bumper to absorb energy in low speed collisions without causing structural damage to the vehicle.